Removal of iron from magnesiumbase alloys



1941- J. D. HANAWALT ETAL 2,267,862

REMOVAL OF IRON FROM MAGNESIUM-BASE ALLOYS Filed Sept. 21, 1940Tempera/we -C 57 U S R 4 M .0? n T wE R my 0 Vflmm ah i? A 5 0 5% Z J (6m Patented Dec. 30, 1941 7 REMOVAL OF IRON FROM MAGNESIUM- BASE ALLOYSJoseph D. Hanawalt, Charles E. Nelson, and

Graydon E. Holdeman, Midland, Mich., assignors to The Dow ChemicalCompany, Midland,

Micln, a corporation of Michigan Application September 21, 1940, SerialNo. 357,788

14 Claims.

This invention relates to the removal of iron impurities from magnesiumand magnesiumbase alloys.

For some time it has been appreciated that the tendency of uncoatedarticles of magnesium and magnesium-base alloys to corrode on exposureto moist air and to aqueous solutions is insome way dependent upon thepresence of traces of iron in the metals. More recently it has beenshown that there is a certain minute but critical proportion of iron, e.g. about 0.002 per cent in the case of magnesium-base alloys containingaluminum, below which these metals are highly corrosion resistant, butabove which they are relatively corrosible. It is evident, then, that inorder to render iron-contaminated magnesium and magnesium-base alloysresistant to attack the iron content thereof must be reduced to belowthe critical corrosion tolerance limit. Unfortunately, however, since atleast part of the iron is in actual physical. solution in the metal whenmolten many of the removal processes described in the art areineffective in reducing the iron concentration to a sufliciently lowvalue. The remaining known processes, while more or less effective, arefraught with operating difiiculties and are wasteful of expensivepurifying agents. Moreover, the ironfree metal produced by suchprocessesusually cannot be remelted and heated to casting temperaturesin iron and steel containers without again becoming contaminated withiron.

It is accordingly an object of'the present invention to provide asimple, convenient, and relatively inexpensive method of treatingironcontaminated magnesium and magnesium-base alloys to reduce theconcentration of iron therein to a proportion well below the criticalcorrosion tolerance limit, 1. e. to below 0.002 'per cent for mostalloys. Another object is to provide a method whereby magnesium andalloys thereof containing less than the critical corrosion limitof ironmay be heated in contact with surfaces of metallic iron at temperaturesup to 925 C; without dissolving iron. Other objects will be apparentfrom the description.

The present invention depends upon the discovery that iron is virtuallyinsoluble, i. e. solu-- ble to an extent well below 0.002 per cent byweight, in molten magnesium and magnesiumbase alloys when these metalscontain manganese in a proportion substantially equal to its solubilitylimit. For instance, when manganese is dissolved in a molteniron-contaminated magnesium alloy in a quantity approximately sufficientto form a saturated solution, the dissolved iron is rendered insolubleand precipitates, until less than 0.002 per cent remains in solution.The resulting alloy, since it contains ironin a proportion below thecritical corrosion limit, is highly corrosion resistant. Likewise, whena substantially iron-free magnesium-base alloy is heated in contact withiron surfaces, if the alloy is nearly saturated with manganese, there islittle if any solution of iron from the surfaces into the metal, sincethe latter is already saturated with iron even though somewhat less than0.002 per cent is present. The corrosion resistance of the alloy is notreduced.

In removing dissolved and suspended iron impurities from magnesium andmagnesium-base alloys according to the invention, any of a variety ofessentially similar procedures may be employed. In general, the metal tobe purified is melted, ordinarily under a suitable protective in aproportion approximately equal to its solubility limit at apredetermined temperature below 925 C., such operation being carried outat a temperature at least as high as the predetermined temperature.Aftef'solution is complete, the temperature of the molten metal is thencontrolled at the predetermined temperasame without agitation,

ture,- whereby a substantially saturated solution of manganese is formedwithout appreciable precipitation of metallic manganese, and thedissolved iron is rendered insoluble and precipitates. This precipitatemay then be separated from the molten metal, usually by maintaining theand preferably also without permitting the temperature to fallappreciably, for a time sufficient topermit the iron to settle out ofsolution. However, other separation methods, such as centrifuging, areentirely possible.

In the invention, .the introduction of manganese into the magnesium ormagnesium-base alloy may be accomplished by any suitable method. Thus,'metallic manganese may be melted together with the magnesium, orstirred into the already molten metal. Alternatively, reduciblemanganese compounds, especially binary compounds such as manganouschloride, manganous fluoride, manganous oxide, manganese dioxide, etc.,may be brought into contact with the molten magnesium,ias byincorporating the compound in the protective melting'flux, whereby thecompound is reduced, forming manganese which dissolves in the magnesium.All these methods are comprehended by the term dissolving manganese inthe metal, as herein e ployed.

The precise proportion of manganese required to form a substantiallysaturated solution with I any given alloy and at any desired separationtemperature may easily be determined by simple experiment. Typicalsolubility data are given in the accompanying drawing, which illustratesgraphically the variation with temperature of the limiting solubility-ofmanganese in molten magnesium, and in certain alloys of magnesium withaluminm. Inasmuch as alloyingelements other than aluminum do not affectthe manganese solubility markedly, the graph will also serve as a roughguide for adding manganese rendered insoluble and precipitating. Themetal is then maintained without agitation fora time sufficient for theprecipitated iron, and also any suspended iron particles initiallypresent as such, to settle out of the metal, usually for 30 minutes ormore to insure complete settling. After this,

the purified metal is separated from the settled iron, as by decanting,and may be. introduced into a casting mold wherein it is allowed tosolidify. As stated, the dissolving, settling, and

casting are all conductedwhile maintaining the metal at a substantiallyconstant temperature, which is usually well above 700 C. to permitsuccessful casting, although temperatures as low as 650 C. are operable.The resulting cast metal contains dissolved ironin' an amount less than0.002 per cent by weight. It may be remelted in iron containers andheated. to temperatures as high as the original operating temperaturewithout dissolving iron from-the container.

In an alternative form of the invention, the

settling and casting operations are carried-out at a substantiallyconstant temperature in the range 650 C. to 925 C., but the dissolvingof manganese is carried out at any desired higher temperature. In thiscase, manganese is incorporated in the heated molten metal in aproportion just suflicient-to form a saturated solution at thesubsequent lower settling temperature, the precise proportion beingdetermined'by the reference to a solubility diagram as explained. Whenthe manganese is completely dissolved in the desired proportion, the.

melt is then cooled to the settling temperature, whereby a substantiallysaturated solution is formed, and the dissolved iron precipitates, andmay be removed by means hereinbefore set forth.

As already stated, the process of the invention depends upon maintainingthe molten magnesium or magnesium-alloy substantially saturated withmanganese at the temperature of settling. In the case of constanttemperature operation, it is sometimes convenient to attain this resultsimply byadding an excess of manganese to the settling vessel. Ingeneral, howe'ver,'it is-highly preferable to avoid the presence of anysubstantial excess undissolved manganese. In this way, waste ofexpensive metal iseliminated and the heating problems resulting-from thepresence of metallic sludge are obviated, and in addition the corrosiveattack by manganese on the iron and steel settling vessels ordinarilyused is largely prevented. To avoid the presence of excess manganese, itis desirable to control quite carefully the quantity of manganese orreducible manganese compound added to the molten metal to be purified soas to approximate closely the satu- In this instance, iron-contaminatedmagnesium is melted under a protective flux, and aluminum is alloyedtherewith in the predetermined proportion. Minor quantities of otheralloying ele-, ments, such as zinc, cadmium, silicon, or tin, may alsobe added if desired. When this operation is completed, manganese or areducible manganese compound is added to the heated melt in a quantitysufiicient to dissolve manganese in the alloy in the predeterminedproportion. The temperature of the resulting alloy is then controlled toapproximately that temperature in the range between about 650 C.'andabout 925 C. at which the solubility limit of manganese in the finalalloy is equal to the proportion of manganese added. In this way, thealloy is rendered substantially, saturated with manganese withoutappreciable precipitation thereof, and the dis solved iron impuritiesbecome insoluble and precipitate, and may be removed ashereinbeforeexplained. .This method of operation, in which the aluminumis added to the magnesium prior t0 the introduction of themanganese, isadvantageous in that the proportion of the aluminum can be carefullycontrolled, as by rapid hardness iron-contaminated tests, withoutinterference by the presence of manganese. It is within the invention,however,

to add the alloying elements, including manganese, in any desired order,or together.

Thus far it has been assumed that inthe process described the dissolvingof manganese and the settling and casting steps are all carried outindirect sequence without allowing the metal to cool below its'saturation temperature. However, there is another procedural sequencewhich.

also utilizes the principle of the invention; In

this latter case, the iron-contaminated magne-- sium or magnesium-basealloy is melted and manganese dissolved therein in a proportionsubstantially equivalent to its solubility limit at any desiredpredetermined temperature above 650 C., preferably in the range 725? C.to 850 C.

The molten metal is then poured directly without a settling period intoan ingot moldand allowed to solidify. The resulting ingot may then bestored .or shipped, and -.remelted for casting at any later-time. Themelt so formed may then be heated without agitation to acastingtemperature' not appreciably above the predeterminedmanganese-saturation temperature.- In

this way, because the magnesiumissaturated with manganese,, theiron-impurities present in settle'out on the bottom of the remeltingpot.

The melt may then be separated from the settled iron. as by decanting,and cast into molds to form finished products containing less than 0.002per cent iron. This method of operation has the advantage that the metalmaybe heated at high temperatures, i. e. up to 925 0., prior to castingthe ingot to efiect a grain refinement, which persists even after theremelting and final casting operations.

In addition-to all the foregoing, of the invention may also .be appliedto those instances in which it is desired to melt purified magnesium ormagnesium-base alloy already containing less than 0.002 per cent byweight of iron in an iron vessel at temperatures between about 700 C.and about 925 C., particularly above 725 C.,'without experiencingcontamination of the purified metal. This result may be attained merelyby dissolving and maintaining in the molten metal a proportion ofmanganese sumcient to form a saturated solution at the temperature ofheating. Because of the manganese saturation, solution of iron from thevessel into the alloy is substantially prevented, and the high corrosionresistance of the purified alloy is not destroyed.

The following examples will serve further to illustrate the invention,but are not to be construed as limiting its scope.

Example 1 Commercial metallic magnesium was melted under a protectiveflux having the following composition in per cent by weight: potassiumchloride, 55; barium chloride, 9; magnesium chloride,

34; calcium fluoride, 2. The molten metal was heated to a temperature of800 C., and metallic aluminum and metallic zinc were then stirred intothe melt in quantities sufficient to form an alloy containing 9.0 percent aluminum and 0.7 per cent zinc. The iron content of this alloy wasfound to be 0.037 per cent. Manganese was then dissolved in the melt inthe proportion of 0.92 per cent, by stirring into the protective flux asufficient quantity of a saline mixture consisting of manganesechloride, ,72 per cent; potassium chloride, 23 per cent; bariumchloride, 2.5 per cent; and calcium fluoride, 2.5 per cent. Theresulting alloy, which was substantially saturated with manganese, wasthen maintained without agitation at 800 C. for two hours, during whichtime the iron impurities, rendered insoluble by the presence ofmanganese, settled out .as a sludge. The purified alloy was thendecanted from the sludge and poured into ingot molds to solidify. Theresulting ingots contained only 0.0007 per cent iron, and were highlyresistant to corrosion by moist air and by salt water. They could beremelted in iron containers at temperatures up to 800 C. or slightlyhigher without any observable increase in iron content.

Example 2 v Commercial magnesium was melted under a protective flux asin Example 1, and heated at a temperature of 785 C. while sufficientfinelydivided metallic manganese was stirredinto the melt to form asolution containing 2.0 per cent by weight of manganese. The resultingalloy, which was found to contain 0.0126 per cent iron, was cooled to700 C., at which temperature it became substantially saturated withmanganese,

. the original alloy remain insoluble, and do not dissolve during theremelting operation, but

the principle e 3 and was held at that temperature without agitation fortwo hours, during which time the iron impurities settled out of thealloy. At the end of the period, less than 0.0007 per cent of ironremained dissolved in the metal.

Example 3 Commercial magnesium, metallic aluminum, and metallicmanganese were melted'together at 700 C. under a protective flux to forman alloy containing 10.0 per cent aluminum and 0.5 per' Example 4Dowmetal R, a magnesium-base alloy contain ing 9.0 per cent aluminum,0.7 per cent zinc, and 0.2 per cent manganese, the balance beingmagnesium, was melted under a protective flux and heated to 800 C.Sufiicient metallic manganese to form a saturated solution at thattemperature was stirred into the molten alloy, which was then 0.0015iron, and exhibited immediately poured into ingot molds and allowedtosolidify. The resulting ingots contained 0.004 per cent of iron asimpurity. The ingots were later remelted under flux in a steel pot andheated slowly without agitation to a, temperature of 3300 C. During theremelting, the iron impurities present settled out of the alloy. Thepurified metal was then decanted from the settled iron and cast intobillets, which were found to contain less than 0.001 per cent iron.

Example 5 Dowmetal H, a magnesium-base alloy containnesium, was meltedunder aprotective flux, and

metallic manganese was dissolved in the metal in a proportioncorresponding to its solubility limit at 750 C. The resulting melt washeated with stirring at temperaturesin the range 890 C. to 925 C. for ashort period, and then immediately cast into ingots, which were found tocontain 0.030 per cent iron as; impurity. The ingots were later remeltedand heated without agitation at, 750 C. for a time suflicient to permitthe iron impurities, which were insoluble at that temperature, to settleout. The settled alloy was then cast into billets. The latter containedthe characteristicgrain refinement resulting from the high temperaturetreatment given.

From the discussion, it will be evident that the invention provides asimple and successful meth- 0d of removing iron impurities frommagnesium and its alloys to values less than its critical. corrosionlimit. The method does not employ an excess of an expensivepurifyingingredient, is not wasteful of heat, andsubstantiallyeliminates corrosion of iron and steel containers employed.

Moreover, the purified alloy may be remelted at casting temperatures iniron pots without an observable increase in iron content. The methodalso provides a particularly convenient way of forming iron-freealuminumand manganesecontaining magnesium-base alloys.

It is to be understood that the foregoing description is illustrativerather than strictly 1imitative, and that the invention is co-extensivein scope with the following claims.

1., Themethod of removing dissolved iron 'impurities fromiron-contaminated molten magnesium and magnesium-base alloys whichcomprises: adding to the, molten metal a suflicient quantity of asubstance selected from the 'class consisting of manganese and reduciblemanganese compounds to form in the molten metal. a solution containingmanganese in a proportion approximately equal to its solubility limit inthe metal at a predetermined temperature between about 650 C. and about925 C., while maintaining the metal at a temperature at least as high asthe predetermined temperature; then controlling the temperature of themetal at approxi-' mately the predetermined temperature, whereby asubstantially saturated solution of manganese in the metal is formedwith'out appreciable precipitation of metallic manganese, and thedissolved iron is rendered insoluble and, precipitates; and separatingthe molten metal from the precipitated iron while still maintainingapproximately the predetermined temperature.

2.- The method of. removing dissolved iron impurities fromiron-contaminated molten mag ne sium and magnesium-base alloys whichcom- P I adding to the molten metal a substance selected'from the classconsisting of manganese.

and reducible manganese compounds in a proportion at least suflicient toform in the metal a solution substantially saturated with manganese,

while-maintaining the metal at a substantially constant operatingtemperature below about 925 0., whereby the dissolved iron is renderedinsoluble and precipitates; and thereafter separating the precipitatediron from the molten metal while it is still at the said operatingtemperature. 3. .The method of removing dissolved iron impurities frommolten magnesium and magnesiumbase alloys which comprises: stirring themolten metal and adding thereto a substance selected from the classconsisting of manganese and reducible manganese compounds in a quantitysufficient to form a solution containing manganese in a proportionapproximatelyequal to its solubility limit in the metal at apredetermined temperature between about 650 C. and about 925 C. whilemaintaining the metal at a temperature at least as high as thepredetermined temperature;

- discontinuing stirring and maintaining the metal without agitation atabout the predetermined temperature, whereby there is formed asubstantially saturated solution of manganese in the metal, and thedissolved iron is rendered insoluble and settles out of the moltenmetal; and separating the purified molten metal from the settled ironwhile still maintaining approximately the predetermined temperature.

4. In a method of forming castings comprising less than 0.002 per centby weight of iron from molten magnesium and magnesium-base alloyscontaining dissolved iron' as an impurity, the steps which include:adding to the molten metal a substance selected from the classconsisting of manganese and reducible manganese compounds in aproportion approximately equal to but not in excess of that required toform in the metal a solution substantially saturated with manganese,

wherein it is allowed to solidify, said dissolving,

separating, and casting operations all being car-.-

. proximately the said "whereby the dissolved iron is rendered insolublethe molten manganese in ried out" at a substantially constanttemperature in the range 925 C.

5. In a method of preparing a substantially iron-free manganeseandaluminum-containing magnesium-base alloy from a. molteniron-contaminated aluminum-containing magnesiumbase alloy, the stepswhich comprise: adding to alloy manganese supplied in a proportionapproximately equal to its solubility limit in the alloy at apredetermined temperature between about 650 C. and about 925 C., saidproportionbeing between about 0.2 and about 2.0

between about 650, C. and about per cent by weight; controlling thetemperature of the alloy to about said predetermined temperature,whereby a substantially saturated solution of manganese in the alloy isformed without appreciable pre ipitation of metallic manganese, and thedissol ed iron is rendered insoluble and precipitates; and separatingthe molten alloy from the precipitated iron while still maintainingapproximately the predetermined temperature.

6. Ina method of preparing a magnesiumbase alloy containing less than0.002 per cent .by weight of iron and comprising aluminum and manganesein predetermined proportions between about 0.2 and about 12 per cent ofaluminum and between about 0.2 and about 2.0 per cent of manganese, thesteps which comprise: forming a molten magnesium-base alloy portions;controlling the temperature of the alloy .thus formed to approximatelythat temperature tion of manganese, whereby there is formed a .moltenalloy substantially saturated with man ganese in the desired proportion,and the dissolved ironis rendered insoluble and precipitates withoutappreciable precipitation of metallic manganese; and separating themolten alloy from the precipitate while still maintaining aptemperature.

7. In a method of preparing a magnesium-base alloy containing less than0.002 per cent by weight of iron and comprising aluminum andpredetermined proportions .between about 0.2 and about 12 per cent ofaluminum'and between about 0.2 and about 2.0

per cent of manganese, the steps which com- I prise: providing a body ofmolten magnesium therewith in the predeand alloying aluminum terminedproportion; adding to the resulting-alloy a substance selected from theclass consisting of manganese and reducible manganese compounds in aquantity suflicient to dissolve m'anganese in the alloy in thepredetermined proportion; controlling the temperature-of the alloy thusformed to perature in the range about 925 manganese termined desiredproportion of manganese, whereby there is formed a molten alloysubstantially saturated with manganese in the desired between about 650C. and C. at which the solubility limit of proportion, and the dissolvediron is rendered insoluble and precipitates; and separating the moltenalloy from the precipitate while still maintaining approximately thesaid temperature. '8. In a method of preparing a magnesium base alloycontaining'less than 0.002 per cent by weight of} iron and comprisingaluminum and. manganese in predetermined proportions between aboutcontainingmanganese and aluminum in the aforesaid proapproximately thattemin the alloy is equal to the prede 0.2 and. about 12 per cent ofproportion; controlling the temperature of the alloy thus formed toapproximately that temperature in the range-between about 650 C. andabout 925 C. at which the solubility limit of manganese in thealloy is'equal to the added proportion of manganese, whereby there is formed amolten alloy substantially saturated with manganese in the desiredproportion, and the dissolved iron is rendered insoluble and,precipitates without appreciable precipitation of metallic manganese;and separating the molten in the alloy in the predetermined v metal asolution containing manganese in a proportion approximatelyequal to itssolubihtylimit at the predetermined remelting temperature,

and thereafter without settling introducing. the

resulting alloy into a mold wherein it is allowed to solidify.

121 In a method of preparing a magnesium base alloy casting containingless than 0.002 per cent by weight of iron from an ingot of anironcontaminated magnesium-base alloy containing manganese in aproportionsubstantially equal to its solubility-limit in the alloy at apredetermined temperature above 700 C.; the steps which comprise meltingthe ingot and heating the same without agitation to a castingtemperaturenot substantially above the predetermined temperature, whereby asubstantially saturated solution of manganese in the alloy is formed andalloy from the precipitate while still maintaining approximately thesaid temperature.

9.'In a method of preparing,

predetermined remelting temperature, and thereafter without settlingintroducing the resulting alloy into a mold wherein it is allowed tosolidify.

10. A process according to claim 9 wherein the molten metal is heated toa temperature sufliciently high to efiect grain refinement prior tointroducing it into the mold. v

11. In a method of preparing, from a molten iron-contaminatedmagnesium-base alloy, an ingot capable of being remelted -and settled inan iron container at a predetermined temperature between about 650 C'.and about 925 C. to form a molten alloy containing less than 0.002percent by weight of iron, the steps which comprise: adding to themolten iron-contaminated alloy a substance selected from the classconsisting of manganese and reducible manganese compounds in aproportion suficient to but not in excess of that required to form inthe a proportion" limit at the from a molten iron-contaminatedmagnesium-base alloy, an inbetween about (00 the iron-impuritiesremain-insoluble and settle out of the molten alloy; separating thealloy from the settled iron without allowing themetal to coolappreciably; and introducing the purified alloy into a mold wherein itis allowed to solidify.

13. In a method of preparing a magnesiumbase alloy casting containingless than 0.002

per cent by weight of iron from an ingot of an iron-contaminatedmagnesium-base alloy containing manganesein a proportion substantiallyequal to its solubility limit in the alloy at apredetermined temperatureabove 700 'C.;' the steps which comprisemelting'the ingot and heatingthe same without agitationto a casting temperature substantially equalto ,the predetermined temperature, whereby a substantially saturatedsolution of manganese in the alloy is formed and the iron-impuritiesremain insoluble and settle out of the molten alloy without appreciableprecipitation of metallic manganese; separating the alloy from thesettled iron without allowing the metal to cool appreciably; andintroducing the purified alloy into a mold wherein it is allowed tosolidify.v I

14. In a process wherein a magnesium-base alloy initially containingless than 0.002 per cent by weight or iron is heated at a temperature C.and about 925 C. in contact with. metallic iron, the method ofpreventing solution of iron in the molten metal whichcompriseslmaintaining therein a proportion of manganese sufflcient toform a substantially -saturated solution at the temperature of heating.

' JOSEPH D. HANAWAL'I'.

CHARLES E. NELSON.

GRAYDON' E. HOLDEMAN.

